1. Field of the Invention
The present invention relates to electrical heating cables that use an electrically resistive heating element in a parallel, constant wattage, zone-type construction.
2. Description of Prior Art
Flexible, elongated electrical heating cables and tapes have been used commercially for many years for heating pipes, tanks, valves, vessels, instruments and for many other applications. The heating cables prevent the freezing of fluids in pipes or equipment, and provide for maintenance of minimum process fluid temperatures as required.
Elongated, parallel heating cables may be defined as assemblies of heating elements, connected in parallel either continuously, which is classified as zoneless, or in discrete zones, classified as zoned. The output or watt density of a parallel cable is basically unchanged regardless of cable length, but is slightly affected by the voltage drop along the parallel circuits forming the power-supplying buses.
There are basically four types of flexible, elongated parallel heating cables in use today. They are:
(1) Zoneless-type, self-limiting PA1 (2) Zone-type, self-limiting PA1 (3) Zoneless-type, constant wattage PA1 (4) Zone-type, constant wattage
Zoneless-type, self-limiting cables are exemplified in U.S. Pat. Nos. 3,861,029; 4,072,848 and 4,459,473. These heaters are generally formed of either positive temperature coefficient (PTC) conductive polymers or semiconductive polycrystalline ceramic chips. The conductive polymers may be extruded to connect two spaced-apart parallel power supplying buses, as shown in U.S. Pat. No. 3,861,029 or may be an elongated strip or strand of conductive polymeric material that is placed in contact with the buses alternately with one bus, then the other, as shown in U.S. Pat. No. 4,459,473. The conductive polymeric elements and buses are then encased in an outer insulating jacket. The semiconductive polycrystalline ceramic heaters are formed by placing multiple ceramic chips in contact with and between two spaced-apart parallel buses at close spacing and then encasing the chips and buses in an electrical insulation as described in U.S. Pat. No. 4,072,848.
Zone-type, self-limiting heating cables are exemplified in U.S. Pat. Nos. 4,117,312 and 4,304,044. In these heaters, semiconductive polycrystalline ceramic chips are used to control or limit the power output of the heating zones that are formed by a resistive wire alloy that is spirally wrapped around two electrically insulated parallel buses and alternately connected to a point where the insulation has been removed from first one wire, then the other at prescribed distances. The chips are located in contact with the buses and the alloy wire or just in contact with the alloy wire, depending on the design. The assembly is then encased in an insulating jacket.
Zoneless-type, constant wattage heaters are exemplified by U.S. Pat. Nos. 2,952,761 and 4,485,297. These heaters typically are comprised of a heating element formed from a conductive coating of graphite or carbon dispersed throughout a non-conductive adhesive vehicle, such as an alkali-stabilized colloidal silica as described in Pat. No. 2,952,761, or a colloidal graphite ink as described in Pat. No. 4,485,297. The pattern for the conductive carbon composition is either printed or otherwise dispersed on an electrically insulating substrate that contains parallel bus strips. The substrate with the conductive carbon composition is then covered with an electrically insulating layer to provide a complete heater.
Zone-type, constant wattage heaters include heating elements generally formed of a metal alloy commonly comprised of nickel, chromium and iron and are exemplified in U.S. Pat. Nos. 3,757,086; 4,037,083, 4,345,368, and 4,392,051. In this class of heaters the metal alloy element is generally a small gauge resistance wire that is spirally wrapped around two parallel electrically insulated buses. The resistance wire makes contact on alternate buses at predetermined intervals where the electrical insulation of the buses has been removed to provide direct electrical contact for the resistance wire with the power-supplying bus. The buses with the resistant wire are then encased in an insulation jacket. U.S. Pat. Nos. 4,345,368 and 4,392,051 disclose the use of a resistance wire placed between and running parallel with the buses. An electrically conductive splice then connects the resistance wire alternately with first one bus, then the other bus. This assembly is then encased in an insulating jacket.
As can be seen in the previous discussion, the prior art parallel, constant wattage, zone-type heating cables have used a metal alloy resistance element to generate the heat produced by the cable. Previous zone-type constant wattage parallel heating cables as exemplified by U.S. Pat. Nos. 3,757,086 and 4,037,083 have used a small alloy wire spirally wrapped around two parallel buses as described earlier. Although the spiral wrapping provided fairly even temperature distribution over the surface of the heating cable, a small wire of 36-42 gauge was necessary to provide a heater with reasonable zone dimensions for standard 120 and 240 volt heating cables. This small gauge wire was rather fragile and, under certain stress induced conditions of voltage and temperature cycling, the small wire would break, rendering that particular zone inoperative.
A cable designed according to U.S. Pat. Nos. 4,345,368 and 4,392,051 reduced the stress breakage of the small gauge wire but due to the design, the heat was concentrated along the longitudinal center line of the heating cable and had poor heat distribution around the surface of the cable which caused the heating element to operate at high temperatures due to poor heat dissipation.
Where carbon elements of any type have been used, they have either been used for self-limiting or for zoneless heaters and have not had application in zone-type, constant wattage cables.
Non-metallic, conductive fibers have been used previously in automotive ignition systems as disclosed in U.S. Pat. No. 4,369,423, which systems work with voltages in excess of 20,000 and are not designed to produce heat, but rather concerns are production of minimal radio frequency noise, withstanding environment rigors and conducting sufficiently to ignite the fuel mixture.